Opinion: with the exemplary restoration of Alvar Aalto's seminal Viipuri/Vyborg Library, Finland has schooled Russia in how to treat its neglected 20th-century buildings. Now they need to restore the rest of the city, says Owen Hatherley.

One of the major buildings by one of the 20th century's five or so most praised architects has been restored after decades of dilapidation and decline.

The Vyborg Municipal Library, designed by Alvar Aalto incrementally through the first half of the 1930s, is of some archaeological significance in Aalto's career as it marks the second major shift in his work.

Beginning as a Classicist, which in Finland in the early 20th century meant a quite open, experimental approach to the tradition, he suddenly switched to a bare, elegant functionalism at the end of the 1920s, in buildings like the Paimio Sanatorium.

At Vyborg – or, as it then was, Viipuri – that functionalism was inflected with local reference. In the Library's public auditorium, Aalto practically created the idiom of moderate, "organic" Scandinavian Modernism, which remains influential to this day in every bit of applied wooden cladding on a modern building.

The restoration, which took nearly two decades, was lavishly funded by the Finnish government and has brought the building back up to the very highest standards. The building, however, is not in Finland.

Viipuri/Vyborg, historically a multicultural but largely Finnish city, was annexed by the Soviet Union at the end of the Second World War. In the process it went from being Finland's second city to being the Russian Federation's 208th largest city.

It is now totally Russian in terms of population, but the architecture that has been left there is so overwhelmingly reminiscent of Helsinki that the disconnect is head-spinning.

With its whitewashed medieval towers, grand Classical office blocks, distinctive National Romantic, early Modernist commercial palaces and clipped, white-walled Functionalist apartments – many of them in a highly distressed state – it looks like you might imagine Helsinki might look after a long, drawn out war. So the restoration of the Vyborg Library makes it somewhat incongruous.

In case there's any doubt, Vyborg Library is a superb building, and it's totally right that it be restored as it has been. Nestled in the city's municipal park, it is long, low and gleaming white, but it is inside that Aalto's personal style comes through.

The main reading room is lit with dozens of round roof lights, so that every minute change in the weather – which changes a lot – transforms the mood of the room. Recently, a finishing touch was added to this room, with replicas of Aalto's curved wood balustrades sweeping almost randomly along the second level of this double-height space.

In the auditorium is a distinctive ceiling, an arbitrary gesture allegedly dictated by acoustics, where a delicate wave of wood – originally created by local shipyard workers – was placed under the roof's concrete lid.

After 1945, the building narrowly avoided being bulked up with stone and ornament in Socialist Realist style, and had an inept but well-meaning restoration in the 1960s. Photographs stored in the Library – which has an entire section devoted to the great man – show the scale of its decline by the early 1990s.

Looking around now, you'd never know that anything had happened, and you'd never know that you weren't in achingly affluent Scandinavia, but for the fact that most of the library books are in Russian.

Vyborg's post-1945 architecture is not a credit to the USSR. There is a good, grandiose, if standardised Socialist Realist railway station, and there's a fine Futurist cafe and shopping centre which appears to be literally falling apart, but that's about it.

However, there's a lot of Finnish functionalism of a very high quality indeed, and apart from Aalto's masterwork, it languishes in much the same melancholy state as then-contemporary Constructivism does in Soviet cities like Kharkiv or Nizhny Novgorod, or in most of Moscow.

The work of Uno Ullberg, the city architect in the 1930s, is one example. Like Aalto, he was a convert to Modernism. His earlier manner, a heavy, articulated Art Nouveau comparable to Saarinen or Mackintosh, was exchanged for a classicised Modernism. Some of this – the Viipurin Panttilaitos Oy office block and Vyborg Art School – has been restored, most of it not. But the difference between these and the work the Finns did with Aalto is telling.

The Art School was restored to become a Vyborg branch of the St Petersburg Hermitage, and the building – which is placed on top of a historic fort and has a beautifully clear view of Vyborg's port from its tall colonnades – was patched up. But some more typical Russian kitsch was also added, with Neoclassical statues placed around the building as if to say "here be culture". The interiors and the exhibitions are nondescript.

Just next to it is the more aggressive, Constructivist-like Flour Mill, designed in 1931 by Erkki Huttenen, which is in so poor a state that to the untrained eye it could be any utilitarian, Soviet port building. The same could be said about Olli Poyry's chic, curved Karjala Insurance Company high-rise in the city centre.

If nothing else, Vyborg tells you that the poor condition of most interwar Modern buildings in Russia or Ukraine isn't anything specifically Soviet or post-Soviet – this is just what International Style buildings look like when they've been forgotten for 50 years.

Looking at the Vyborg Library, you can only hope that something similarly generous, scrupulous, thorough and complete happens to more of the neglected masterpieces of the 20th century – from Le Corbusier's work in Chandigarh to Moisei Ginzburg's Narkomfin building to Robin Hood Gardens (at least to test the questionable theory that it is a masterpiece).

It is also gratifying that this Municipal Library remains just that, and a good one. But restoring a whole city – and Vyborg's architecture is rich and important enough to deserve that – is a much more difficult problem than restoring one great building by a very famous architect.

Owen Hatherley is a critic and author, focusing on architecture, politics and culture. His books include Militant Modernism (2009), A Guide to the New Ruins of Great Britain (2010), A New Kind of Bleak: Journeys Through Urban Britain (2012) and The Ministry of Nostalgia (2016).